A video is a sequence of at least one image. Indeed, an image can be considered as a still video. A video is encoded on a rectangular frame that is a two-dimension array of pixels (i.e. element of color information). One frame is encoded per image of the sequence. An image is encoded according to a mapping function. Legacy videos, meant to be displayed on a rectangular screen are encoded according to the rectangle mapping. Immersive videos are meant to be rendered around the viewer, i.e. the viewer is not able to see the entire picture but has to rotate his/her head (or to move or to use a controller, e.g. a joystick or a mouse) to see parts of the image out of his/her field of view. Immersive videos, when encoded on a rectangular frame, require an immersive mapping function, for example, equirectangular mapping, cube mapping or pyramidal mapping.
A video stream is prepared and formatted according to the type of rendering device that the video is meant to be rendered on. Legacy video rendering devices do not properly display immersive videos as they are configured to decode video frames only according to the rectangle mapping. Reciprocally, immersive video rendering devices do not properly display legacy videos as they expect frames encoded according to an immersive projection. There is a lack of a format for a backward compatible immersive video stream which could be properly displayed by both legacy and immersive video rendering devices.